1. Field of the Invention
This invention relates to a selective ion etching method and, more particularly, to a method for selectively etching, with a high etching selectivity and without employing a Freon-based gas, a layer of a III-V group compound semiconductor essentially free from Al, such as GaAs compound semiconductor, which has been formed on an Al-containing III-V group compound semiconductor, such as AlGaAs compound semiconductor. More particularly, it relates to a method for producing a field effect transistor, such as high electron mobility transistor (HEMT) or MES-FET, by taking advantage of the selective etching with a high selectivity ratio.
2. Prior Art
Compound semiconductors, such as GaAs, are finding a widespread use in recent days because of the higher electron mobility proper to the compound semiconductor than that achieved with silicon prevalently used as a semiconductor. The high-speed field effect transistor (FET) making use of the compound semiconductor has now become the fundamental, technique in the field of high speed communication and electronic computers.
Meanwhile, when preparing a HEMT or MES-FET using a GaAs substrate, formation of a gate electrode is a step which is most critical throughout the entire production process. That is, the characteristics of the device depend on the shape or the size of the gate electrode. Above all, if the gate length is reduced to a minute size in search for an elevated operating speed, it becomes most critical to produce the gate electrode with high reproductivity.
As a technique of forming a gate electrode in a field effect transistor making use of the GaAs substrate, there has so far been known a technique of employing a dummy gate as described in Japanses Patent Publication laid-open No.62-60269. There is also known a technique of performing a recess etching after which the gate electrode is formed. If the gate electrode is formed after the recess etching, there results a lowered RS value.
In conventional recess etching, wet etching using a phosphate-based material or a dry etching using a CCl.sub.2 F.sub.2 /He type gas has been employed. In carrying out such recess etching by the conventional HEMT dry etching, an n-AlGaAs layer, as an electron donor layer, is formed on an undoped GaAs layer, an interface of which functions as a two-dimensional electron channel layer. A GaAs layer, as a cap layer, is then formed on the n-AlGaAs layer. The GaAs layer is then removed by recess etching with a high selectivity ratio and the gate electrode is subsequently formed.
However, if wet etching or dry etching using a CCl.sub.2 F.sub.2 /He gas is used by way of the above-mentioned recess etching, the following problems ar raised.
That is, if a recess is to be formed by wet etching, the recess can be formed only with poor uniformity or reproductivity, such that it is difficult to produce elements with constant characteristics.
On the other hand, if the recess is to be formed by dry etching using the CCl.sub.2 F.sub.2 gas, since the CCl.sub.2 F.sub.2 gas itself is among gases placed under the regulation of the Freon gases, use of a large quantity of the gas leads to destruction of the environment of the earth. SiCl.sub.4 /SF.sub.6 base gases or Cl.sub.2 /SF.sub.6 gases, proposed as alternative gas for the CCl.sub.2 F.sub.2 /He gas, are inconvenient in that the gases may be deposited on high pressure regions.